Conventionally, displays of the back-lighting type have been widely used as advertising signs and road-traffic signs in order to make them discernible at night or in order to improve visibility of the contents of the displays such as characters and diagrams. In these displays, patterned display boards, having the contents of displays, are made with a light-transmitting or semi-light-transmitting property, and the information of the displays is visually displayed by illuminating the patterned display boards from behind. Plane-shaped lighting devices, which are used as back lights for such displays, are mainly classified into two systems, the edge-light system and the direct under-light system.
For example, as illustrated in FIG. 50, a plane-type lighting device of the edge-light system, which is placed on the back of a patterned display board in parallel therewith, has a light-directing plate 1 made of a plate-shape light-transmitting member and a light source 2 that is placed at a light-incident surface 1a that forms one side end of the light-directing plate 1. Light, released from the light source 2, is made to be incident on the light-incident surface 1a of the light-directing plate 1. Then, the light, which has proceeded inside the light-directing plate 1 after repeating total reflections, is taken out ahead of a plane section 3 by a light-takeoff body 4 that is provided on the rear surface 3b or the front surface 3a of the plane section 3, and made to be incident on the patterned display board. In contrast, in a plane-type lighting device of the direct-under system, a rod-shaped light source such as a fluorescent lamp is placed on the back of a patterned display board so as to directly illuminate the patterned display board. Here, since the direct-under system needs to have more thickness than the edge-light system, the edge-light system has been widely used for plate-shaped lighting devices that require thin structures.
The following description will discuss the plane-shaped lighting device of the edge-light system. The light-directing plate 1 is made of, for example, a light-transmitting material such as acryl, and the light source 2 is constituted of, for example, rod-shaped or array-shaped light sources such as fluorescent lamps or LED lamps. Further, as illustrated in FIGS. 51(a) and 51(b), light-takeoff bodies 4 are formed on the front surface 3a or the rear surface 3b of the plane section 3 of the light-directing plate 1 so as to form, for example, a dot-patterned or circle-patterned scattering surface. The density of the light-takeoff bodies 4 increases as they are located apart from the light source 2. The light-takeoff bodies are provided as scattering print surfaces 4A that have been formed by using white-based ink or other materials by a screen printing method or other methods, as shown in FIG. 51(a), or provided as rough surfaces 4B that have been formed by roughening the surface of the light-directing plate 1 by using the blast treatment or other treatments, as shown in FIG. 51(b). Thus, light that has been made to be incident on these bodies is allowed to scatter in all directions, and is taken out from the light-directing plate 1.
Moreover, a reflection plate 5 is placed on the rear surface 3b of the plane section 3 so as to keep the scattered light from going out in the rear-surface 3b direction. Furthermore, for example, reflection layers, (not shown), made of materials such as reflective tapes and white ink, are disposed on the side faces 1b, 1c and 1d other than the light-incident surface 1a of the light-directing plate 1 so that the light that has proceeded inside the light-directing plate 1 without having been taken out from the plane section 3 of the light-directing plate 1 is again returned into the light-directing plate 1.
FIG. 52 shows another prior art example of a plate-shaped lighting device. The structure of this plate-shaped lighting device, which is similar to that illustrated in FIG. 51, is provided with light-takeoff bodies 4, each of which has either a solid-line shape or a broken-line shape and is formed on the rear surface 3b of the plane section 3 of the light-directing plate 1, as shown in FIG. 53. The light-takeoff bodies 4, which form a plurality of line-shaped grooves 4C parallel to the light-incident surface 1a, have their density increased as they are located apart from the light source 2. The grooves 4C are formed into a V-letter shape by machining the rear surface 3b of the light-directing plate 1 by the use of, for example, a dicing method or an etching method. Here, the light that has been made to be incident on each groove 4C is reflected toward the plane section 3, and taken out from the plane section 3 through a scattering sheet 6.
In the plane-shaped lighting devices of the edge-light system using the light-directing plate 1, LED lamps 7 have been used as the light source particularly in order to provide low power consumption and maintenance-free operation. As illustrated in FIG. 54, a plurality of LED lamps 7, which forms a light source 2, are arranged in an array on a substrate 8 in a manner opposing the light-incident surface 1a of the light-directing plate 1. As illustrated in FIGS. 55(a) and 55(b), in each LED lamp 7, a lead frame 11 with a reflection cup 10 inside of which an LED chip 12 is installed, and the LED chip 12 is molded with a light-transmitting resin 13. Further, a semispherical lens 15 is formed at one portion of a box-like body 14, and the LED lamps are classified into two types, that is, the scattering type and the transparent type, depending on the types of molding resins that constitute the box-like body 14.
In the LED lamp 7 of the scattering type, shown in FIG. 55(a), since it is molded with light-transmitting resin 13 to which filler is added, light emitted from the LED chip 12 is scattered by the filler inside the resin 13, and transmitted in all directions. In contrast, the LED lamp 7 of the transparent type, shown in FIG. 55(b), is formed by using transparent resin as the molding resin 13 and is normally designed as a high-luminance lamp. Further, in order to converge light in the light-axis direction, the distance Z from the LED chip 12 to the top of the semispherical lens 15 is set to be 1.8 to 2.1 times as long as the radius of curvature of the semispherical lens 15.
The use of the plate-shaped lighting devices of the edge-light system has made display devices thinner. In such display devices of the back-lighting type, a patterned display board, which displays information such as diagrams and characters, has a construction wherein its pattern is displayed by changing transmitting colors and transmittances through character portions and their surrounding portions. Further, in some specific cases of such a construction, the transmittance of either the character portions or the surrounding portions is set to zero, and the character portions or their peripheral portions are subjected to a cut-out process so that predetermined light-transmitting color filters can be provided at the portions that transmit light.
Japanese Laid-Open Patent Publication No. 85588/1992 (Tokukaihei 4-85588) has disclosed one of such displays wherein the character portions are subjected to the cut-out process and color filters are provided at the cut-out portions. As illustrated in FIG. 56, LEDs 21, which serve as a light source, are arranged in series with one another on a side-end face 20c of a light-directing plate 20. Further, a reflection plate 22 is placed on the rear surface of the light-directing plate 20; a light-scattering sheet 23 is affixed onto the front surface of the light-directing plate 20; and a patterned display board 24 is further placed in front thereof. The patterned display board 24 has a structure wherein a light-shielding plate 26 having a pattern 26a formed in a cut-out fashion is superimposed on a color filter 25 that transmits the same color as that of the light-emission from the LED 21. In most cases, in the patterned display board 24, those character portions are formed by using methods, such as screen printing, lamination of cut-out patterns made by etching, and spray coating by masking.
In this arrangement, light emitted from the LED 21 is taken out ahead of the light-directing plate 20 through the inside of the light-directing plate 20. The light, which has been released ahead of the light-directing plate 20, illuminates the patterned display board 24 that is placed in front thereof from behind through the light-scattering sheet 23 that is placed in front of the light-directing plate 20 and is projected ahead of the display device through the color filter 25 and the pattern 26a of the light-shielding plate 26.
In the following description, consideration will be given on the incidence of light in the above-mentioned plate-shaped lighting device. When LED lamps 7 of the light-scattering type are used, the rate at which light rays emitted from the LED lamps 7 are directed directly to the light-incident surface 1a of the light-directing plate 1 is extremely low, consequently resulting in a poor incidence efficiency. Further, in the case of LED lamps 7 of the transparent type, light rays emitted from the LED chips 12 have most of the rays released laterally from the side faces of the box-like body 14 without being converged by the semispherical lens 15 when they have angles not less than .theta.=35.degree. with respect to the light axis, or released laterally after having been total-reflected by the side faces of the box-like body 14, thereby failing to provide a good incidence efficiency for the light rays that are directed directly to the light-incident surface 1a of the light-directing plate 1. The following methods have been proposed as solutions for improving the incidence efficiency.
1 As illustrated in FIG. 57, a housing 30, which covers the periphery of the LED lamp 7, is installed and a reflection layer 31 is formed on the surface thereof. Thus, light rays laterally released from the LED lamp 7 are reflected and directed to the light-directing plate 1. Normally, even those materials such as plastic, which have comparatively high reflectivities, only have approximately 70% reflectivity, and light attenuates to a great degree after merely having been subjected to several reflections. Therefore, this method fails to provide practical effects. Here, the reflectivity of not less than 90% is available by applying a reflection-increasing coat onto the surface of the housing 30. However, since the reflection-increasing coat is formed by the vacuum vapor deposition method, high costs become a problem.
2 As illustrated in FIG. 58, a hole 32 or a groove is formed in the light-incident surface 1a of the light-directing plate 1, and the LED lamp 7 is inserted into the hole 32. In this case, light rays laterally released from the side faces of the LED lamp 7 are also allowed to be temporarily incident on the light-directing plate 1. However, since the progressing directions of the light rays have not been changed, the light rays are not transmitted inside the light-directing plate 1 through total reflections; thus, this method also fails to provide an effective solution. Moreover, with respect to large light-directing plates having a width of several tens of centimeters, since it is difficult to provide them through a molding process, they have to be provided by using a cutting work. This makes the processing costs very expensive.
3 As illustrated in FIG. 59, a hole 32 is formed in the light-incident surface 1a of the light-directing plate 1, and after the LED lamp 7 has been inserted into the hole 32, the space between the LED lamp 7 and the light-directing plate 1 is filled with resin 33. In this case, optical interfaces no longer exist between the LED lamp 7 and the light-directing plate 1, and since total reflections inside the LED lamp 7 and refractions between the interfaces are eliminated, light rays released from the LED chip 12 are improved in their effective angle range by about 10.degree. with respect to the light axis. Further, when the light rays pass through the interface, the Fresnel's reflection is also reduced by several percent. However, this method also makes the costs of processing, assembling and other processes become expensive.
In the following description, consideration will be given on the takeoff process of light in the above-mentioned plate-shaped lighting device. The light, which has been made to be incident on the light-incident surface 1a of the light-directing plate 1, progresses inside the light-directing plate 1 while repeating total reflections on the border faces between the light-directing plate 1 and air. The light thus directed is scattered by the light-takeoff body 4, for example, the scattering print surfaces 4A or the rough surfaces 4B, that is installed on the front surface 3a or the rear surface 3b of the plate section 3 of the light-directing plate 1, or reflected by the V-shaped grooves 4C. Consequently, the directed light is taken out from the plane section 3 of the light-directing plate 1.
However, as shown in FIGS. 51(a) and 51(b), since this arrangement aims to take out the light rays that have impinged onto the scattering print surfaces 4A or the rough surfaces 4B by utilizing the scattering of light, all the light rays are not necessarily taken out from the light-directing plate 1 since the light rays scatter in all directions; one portion of the light rays is again enclosed inside the light-directing plate 1. Although some of the enclosed light rays impinge onto the scattering print surfaces 4A or the rough surfaces 4B and are again taken out from the light-directing plate 1, the rate at which the light rays progress inside the light-directing plate 1 and impinge onto respective side faces 1a, 1b, 1c and 1d of the light-directing plate 1, is increased. Among the light rays that impinge onto the side faces 1a, 1b, 1c and 1d, those light rays that impinge onto the side faces 1b, 1c and 1d having reflection layers are attenuated due to the reflectivity of the reflection layers, and the light ray that impinges onto the light-incident surf ace 1a opposing the light source 2 passes through the light-incident surface la without being taken out from the plane section 3 of the light-directing plate 1. As described above, the problem with the light-directing plate 1 using the scattering print surfaces 4A or the rough surfaces 4B is that the efficiency in the light to be taken out from the plane section 3 of the light-directing plate 1 is deteriorated.
In contrast, in the case of the V-shaped grooves 4C installed on the rear surface 3b of the light-directing plate 1 as shown in FIG. 53, the light rays that impinge onto the V-shaped grooves 4C are taken out from the plane section 3 of the light-directing plate 1 by reflections; therefore, the efficiency in the light to be taken out from the plane section 3 of the light-directing plate 1 is increased, compared with the case where the scattering print surfaces 4A or the rough surfaces 4B are installed. However, since the V-shaped grooves 4C respectively serve as line-shaped mirrors, virtual images 41 of the light source 2 are respectively formed within the virtual-image range 40 of the light source 2, as illustrated in FIG. 60. For this reason, in the case when the LED lamps 7 are used as the light source 2, since the LED lamps 7 serve as a dot-shaped light-emitting source, the respective adjacent virtual images 41, that is, the virtual images 41 of the respective dots of the LEP lamps 7, are aligned in a straight line within the virtual-image range 40. As a whole, the virtual images 41 appear to be a vertically striped pattern, thereby causing bright-and-dark distributions on the plane section 3. This has presented a problem of poor uniformity in luminance. Further, since the virtual images 41 thus formed tilt with respect to the light-directing plate 1, the further the point in question is apart from the light source 2, the further the virtual images 41 are located from the rear surface 3b of the light-directing plate 1. Consequently, when the plane section 3 of the light-directing plate 1 is viewed diagonally with respect to the frontal direction, there appears a portion at a corner section 42 (indicated by a shaded portion in the drawing) apart from the light source 2 of the light-directing plate 1, where the virtual images 41 of the light source 2 become invisible. This has presented a problem of poor uniformity in luminance in the plane section 3 of the light-directing plate 1.
Furthermore, even in the case when the light source 2 is not the dot-shaped light source, but a light source that has no bright-and-dark distributions, such as fluorescent lamps, it becomes possible to eliminate the stripe-shaped bright-and-dark distributions of the virtual images 41 within the virtual-image range 40. However, this arrangement still fails to solve the problem as described earlier, wherein when viewed diagonally, there appears a portion at the corner section 42 apart from the light source 2, where the virtual images 41 of the light source 2 become invisible. In order to solve the problem of poor uniformity in luminance within the plane section 3, a scattering sheet 6 is provided on the plane section 3. However, the installation of the scattering sheet 6 that scatters the light rays impinging thereon also causes the take-off efficiency of light to become worse.
As described above, in terms of light-incidence efficiency, the problems with the conventional plate-shaped lighting devices are a poor efficiency and high costs required for improving the efficiency. Also in terms of takeoff efficiency of light, the problems are poor efficiency and irregularities in luminance on the light taken out. Therefore, one of the objectives of the present invention is to provide a plate-shaped lighting device which achieves uniform luminance by improving the efficiency of light utilization, that is, by increasing light-incidence efficiency as well as light-takeoff efficiency.
Another problem with display devices using the above-mentioned plate-shape lighting device is that only poor visibility can be achieved due to the inefficiency of light utilization of the plate-shaped lighting device. The other problem is as follows: character portions, provided on a patterned display board, are displayed by light that passes through color filters of absorption type that contain coloring matters, such as pigments, that optically absorb predetermined colors. Upon passing through one of the color filters, the light that has taken out from the light-directing plate merely has a transmittance of approximately 75 to 80% at a thickness of approximately 1 mm particularly when its setting of the color transmission is placed on the wavelength side shorter than green, even if the transmitting wavelength band of the color filter is properly coincident with the wavelength of light emitted from the light source. This reduces the efficiency of light utilization. Moreover, in the case of thick light-directing plates, the light transmittance further decreases, thereby reducing the efficiency of light use. This results in poor contrast between the character portions and the surrounding portions of the character portions.
Furthermore, in the case when a display device is illuminated by natural light during day time or external artificial light (such as light from head lamps of a car or a flash lamp), that is, in the case when the display is viewed through reflections of external light, among the light rays that have reached the color filter of the character portions from outside the display device, those light rays that coincide with the setting of the color transmission of the color filter are allowed to pass with the same transmittance as described above (75 to 80% at a maximum at approximately 1 mm) and reach the light-directing plate on the rear side. The light rays that have reached the light-directing plate pass through the light-directing plate from the surface to the rear surface, or are diffracted or absorbed by the light-takeoff body on the light-directing plate. The light rays that have passed through the light-directing plate are further reflected or scattered by the reflection plate that is placed at the back-surface of the light-directing plate, and again allowed to pass through the light-directing plate from the rear surface to the surface, or diffracted or absorbed by the light-takeoff body on the light-directing plate. Thus, the resulting light rays are externally released with the same transmittance as that obtained upon incidence from outside. Therefore, since the transmittance of the color filter (with a thickness of 1 mm) is 75 to 80%, the light rays that have been made to be incident onto the color filter of the character portions from outside have a transmittance of 56% to 64% after having reciprocally passed through the color filter, which results in a great light loss. This further results in reduced contrast (reduction in the difference between luminances) between the character portions and the surrounding portions of the character portions, thereby causing low visibility.
Furthermore, in the arrangement of conventional display devices, the colors in the character portions are of course determined and fixed by tones (light-transmitting characteristics) of the filters of the patterned display board when viewed under external light, and the tones that are transmitted through the color filters are also fixed when viewed after having turned their light sources on. Therefore, it is not possible to change the tones of the display patterns without conducting a physical change such as replacement of the color filters installed on the light-transmitting portions of the patterned display board to other color filters. In addition, the contents of display on the display device are limited to one kind, and it is not possible to display different contents without conducting a physical change such as replacement to another patterned display board having light-transmitting portions with a different pattern.
Moreover, the manufacturing method of the patterned display boards was complicated as described earlier so that a lot of costs and time were required for producing the patterned display boards. Therefore, it was impossible to easily change the contents of display in response to a change in operational circumstances or an alternation in usage.
Accordingly, the objectives of the present invention are to use a plate-shaped lighting device with high efficiency of light utilization, and also to provide a thin display device that has high visibility by improving the efficiency of light utilization.